Yoghurt is a form of fermented milk curdled to a smooth creamy or custard-like consistency by lactic acid-producing microorganisms. The production of natural yoghurt entails the innoculation of a fluid milk substrate, which may contain other components, for example, stabilizers and sugar, with a beneficial culture of Lactobacillus bulgaricus and Streptocacuus Thermophilus followed by incubation at a temperature of about 100.degree.-110.degree. F. until the pH of the substrate is reduced to about 4.5 to 4.3 which indicates that conversion to the desired product, yoghurt, has been completed. The yoghurt is then cooled to inhibit or prevent growth of undesirable microflora. The milk substrate, and the natural yoghurt produced have a solids content of about 11-13% by weight. The incubation may take place in the individual containers in which the yoghurt is to be sold or may take place in a vat or similar vessel and when complete, the yoghurt transferred into the individual containers. Fruit, fruit preserves or other flavourings may be added to the plain yoghurt base. The increase in yoghurt consumption in recent years, especially in North America, has been quite phenomenal (for example, between 1960 and 1975 per capita consumption in the United States increased 483%). Because of the greatly increased demand, yoghurt is now produced on a large scale and sold in a wide variety of retail outlets, including supermarkets, etc. However, the product is highly perishable and requires refrigeration if it is to be kept for even limited periods of time; the maximum shelf life for a natural yoghurt under refrigerated conditions being only several weeks. Consequently, it may only be transported to a limited extent. There has, therefore, not surprisingly been many attempts to treat yoghurt so as to extend its shelf life and also render refrigeration unnecessary. Those attempts have taken many forms. For example, Ferguson in U.S. Pat. No. 3,080,236, produces an instant yoghurt which requires no refrigeration and has a long shelf life by implanting a culture in whole or skimmed milk having the used milk solids content and following incubation at about 100.degree. F. for 12 hours, the yoghurt so-produced is reduced in volume by evaporation and then substantially dried in vacuum driers. That product, when powdered is admixed with a variety of other components including a minimum of 10% of an instant water soluble starch, which is needed to provide sufficient body to the reconstituted yoghurt, and a fat to give the product the appearance and taste of yoghurt. Schut in U.S. Pat. No. 4,066,794 discloses a similar instant yoghurt preparation where the "body" in the reconstituted yoghurt is provided by a greater than 5% by weight of sodium alginate, a gelling or settling agent which is said to impart a curd-like consistency having thixotropic qualities to the reconstituted product, as well as almost 70% by weight of sugar. An alternative route to a yoghurt having an extended shelf life is taught by Egli et al in U.S. Pat. No. 3,932,680. That patent discloses a process which produces a sterile yoghurt. In that process, pasteurized and homogenized milk is concentrated 10%; 10% to 12% of sugar is added; the mixture cooled and incubation to a pH of 4.0 to 4.3 is effected using cultures of lactic acid bacteria over a period of 2.5 to 4 hours at a temperature of from 42.degree.-44.degree. C., the resulting yoghurt base being cooled to between 30.degree.-20.degree. C. Subsequently, it is further cooled to a temperature of from 12.degree.-4.degree. C. when an effective amount of the following stabilizers are added:
Stabilizer A which contains by weight, 20-30% emulsifier; 20-40% carrageenan; 30-40% carboxymethylcellulose; 2-3% carobbean meat or powder; and 4-8% guar meal; PA0 Stabilizer B which is potato starch or other starch; and PA0 Stabilizer C which contains by weight 75-85% of 60% modified gelatin and sucrose (the carrier for the dissolving or distribution of protein.
The resulting mass is then allowed to stand at 4.degree.-12.degree. C. for 5-7 hours when, presumedly the yoghurt has been produced. The yoghurt is then introduced into containers which are hermetically sealed and subjected to a sterilizing procedure involving heating the containers in an autoclave at 60.degree.-85.degree. C. and a pressure of up to 2 atmospheres for a time sufficient to sterilize the product and the packages, cooling under pressure to a temperature of from 10.degree.-15.degree. C. and, finally, storing the packages for 2 to 4 days at a temperature of from 4.degree. to 6.degree. C. To say the least, the described process is extremely complex and time consuming, most probably to the extent that the process cannot be worked economically on an industrial scale. In fact, applicant believes that there are no sterile true yoghurt products on sale, at least in North America, at the present time (several attempts to market such products over the last several years having failed). Reference may be made once again to the relatively high sugar content and also to the fact that cooling prior to the thermal treatment is essential if syneresis of whey in the final product is to be avoided. A modified embodiment of the invention of U.S. Pat. No. 3,932,680 is described in U.S. Pat. No. 4,235,934.
There are many problems associated with producing a thermally treated sterile yoghurt, the most important from a consumer acceptance viewpoint basic difficulty being that the yoghurt body or texture consists of a delicate protein matrix which can easily be disrupted and destroyed by heat and also by mechanical action such as pumping or the like required in a commercial processing operation. In addition, heat treatment can result in the yoghurt's characteristic flavour can be seriously degraded to the extent that in many prior art thermally treated yoghurt products, following the thermal treatment, it is necessary to include a component which imparts a "yoghurt flavour" to the final product. Consequently, it is not possible merely to sterilize standard yoghurt and obtain a product that will be acceptable to the consumer. As the prior art extensively demonstrates, the retention of yoghurt taste and texture is essential if an acceptable product is to be obtained and the prior art products, leave much to be desired in one, or generally both, of those characteristics.
It is an object of the present invention to economically provide a sterile or ascept natural yoghurt having the taste and texture of natural yoghurt and which has an extended shelf life without refrigeration.